Half my life....

server.R

half_my_life <- function(birth = "1978-09-05", event_date = "1997-05-30") {
  birth <- as.Date(birth)
  event_date <- as.Date(event_date)
  if (event_date <= birth) stop("You've got your dates mixed up....")
  Duration <- (event_date - birth) * 2
  Date <- birth + (Duration)
  FDate <- format(Date, "%A %d %B %Y")
  Age <- as.numeric(Duration)/365.242
  if (Date < Sys.Date()) {
    String <- c("You were able to say that this event has been half your life on ",
                "\n%s, \n", "at which point you would have been about \n%02f years old.\n\n")
  } else {
    String <- c("You will be able to say this event has been half of your life on ", "\n%s, \n", 
                "at which point you would be about \n%.02f years old.\n\n")
  }
  sprintf(paste(String, collapse = ""), FDate, Age)
#   return(invisible(list(Target = Date, Target_Formatted = FDate, Age = Age)))
}

# server.R

library(shiny)

shinyServer(function(input, output) {
  output$summary <- renderText({
    input$goButton
    paste0('The start date specified is ', format(input$birth, "%d %B %Y"),
           ', and the event date specified is ', format(input$event, "%d %B %Y"), ".")
  })
  output$value <- renderText({
    half_my_life(input$birth, input$event)
  })
})

ui.R

# ui.R
library(shiny)

shinyUI(fluidPage(
  titlePanel("Half my life"),
  
  sidebarLayout(
    sidebarPanel(
      dateInput("birth", 
                label = h3("Birth"), 
                value = "1978-09-05"),
      hr(),
      dateInput("event", 
                label = h3("Event Date"), 
                value = "1997-05-30"),
      br(),
      actionButton("goButton", "Go!")
    ),
    mainPanel(
      h4("Input Values"),
      textOutput("summary"),
      h4("Results"),
      textOutput("value")
    )
  )
))